1. Field of the Invention
The present invention relates to a method of inhibiting the corrosion of and the formation of scale deposits on metallic surfaces of water-carrying systems, particularly, with regard to corrosion, where the water of the system is oxygen-bearing. More particularly, the present invention relates to the use of compositions comprising amine adducts of polymaleic anhydride to inhibit the corrosion of and the formation of scale deposits on metallic surfaces of water-carrying systems. Most particularly, the present invention concerns the use of compositions comprising amine adducts of polymaleic anhydride together with zinc to inhibit the corrosion of metallic surfaces of aqueous systems.
The term "aqueous", as used herein, is intended to describe water in any physical state and to include water in which is dissolved or dispersed any substance, for example, inorganic salts in brine or seawater.
The term "metallic", as used herein, is intended to include metallic and metal-containing materials comprising ferrous, non-ferrous or alloy metal compositions.
Polymaleic anhydride, as used herein, is intended to include hydrolyzed polymaleic anhydride, which is essentially polymaleic acid. Under most ambient conditions, such hydrolysis to the acid form will take place.
Generally, scale deposits are incrustation coatings which may be formed from a wide variety of simple and complex inorganic salts which accumulate on the metallic surfaces of a water-carrying system through a number of different causes. While the method and compositions of the present invention have been found particularly useful in providing inhibition of calcium carbonate and calcium sulfate scales, inhibition of magnesium hydroxide, calcium fluoride, calcium phosphate, and other common scales may also be obtained. Various industrial and commercial water-carrying systems are subject to scale formation problems. Scale is of particular concern in heat exchange systems employing water, such as, for example, boiler systems, and once-through and open recirculating water cooling systems.
The water employed in these systems ordinarily will contain a number of dissolved salts, the amount and nature of which will, of course, depend upon the source of the water employed. Thus, the water usually contains alkaline earth metal cations, primarily calcium and magnesium, and such anions as bicarbonate, carbonate, sulfate, silicate, phosphate, oxalate, fluoride, and so forth. Combination products of these anions and cations will precipitate from the water in which they are carried to form scale deposits when the concentration of the anion and cation comprising the combination or reaction product exceed the solubility of the reaction product. Thus, when the concentrations of calcium ion and carbonate ion exceed the solubility of the calcium carbonate reaction product, a solid phase of calcium carbonate will form as a precipitate. Precipitation of the reaction product will continue until the solubility product concentrations of the constituent ions are no longer exceeded.
Numerous factors may be responsible for producing a condition of supersaturation for a particular reaction product. Among such factors are changes in the pH of the water system, evaporation of the water phase, rate of heat transfer, amount of dissolved solids, and changes in the temperature or pressure of the system.
For boiler systems and similar heat exchange systems, the mechanism of scale formation is apparently one of crystallization of scale-forming salts from a solution which is locally supersaturated in the region adjacent the heating surface of the system. The thin viscous film of water in this region tends to become more concentrated than the remainder of the solution outside this region. As a result, the solubility of the scale-forming salt reaction product is first exceeded in this thin film, and crystallization of scale results directly on the heating surface.
In addition to this, a common source of scale in boiler systems is the breakdown of calcium bicarbonate to form calcium carbonate, water and carbon dioxide under the influence of heat. For open recirculating cooling water systems, in which a cooling tower, spray pond, evaporative condensor, and the like serve to dissipate heat by evaporation of water, the chief factor which promotes scale formation is concentration of solids dissolved in the water by repeated evaporation of portions of the water phase. Thus, even a water which is not scale forming on a once-through basis usually will become scale forming when concentrated two, four, or six times.
The formation of scale deposits poses a serious problem in a number of regards. The different types of scale which are formed all possess a low degree of heat conductivity. Thus, a scale deposit is essentially an insulating layer imposed across the path of heat travel from whatever source to the water of the system. In the case of a boiler system, the retarded heat transfer causes a loss in boiler efficiency. Increased input of heat to compensate for this loss results in overheating of the boiler metal and consequent tube failures. In addition to this problem, scale formation facilitates corrosive processes, and a substantial scale deposit will interfere materially with fluid flow. Consequently, scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal.
Corrosion of the metallic surfaces of a water-carrying system consists of the destruction of the metal by chemical or electrochemical reaction of the metal with its immediate environment.
Where the corrosion is electrochemical in nature, a transfer or exchange of electrons is necessary for the corrosion reaction to proceed. When corrosion of the metal takes place, two partial electrochemical processes occur, and must occur, simultaneously. There is an anodic oxidation reaction in which metal ions go into solution, leaving behind electrons; and a cathodic reduction reaction in which species in solution are reduced by consuming the electrons produced by the anodic reaction. Where the metal is ferrous or ferrous-containing, and the water system contains oxygen, these two processes may be illustrated by the following equations:
Anodic oxidation: Fe .fwdarw. Fe.sup.+ .sup.+ + 2e.sup.- PA1 Cathodic reduction: 2H.sub.2 O + O.sub.2 + 4e.sup.- .fwdarw. 4OH.sup.-
The two ionic reaction products, ferrous ion and hydroxyl ion, combine to form ferrous hydroxide, Fe(OH).sub.2, which is then oxidized to form rust, ferric hydroxide, Fe(OH).sub.3. For ferrous or ferrous-containing as well as other metals in water systems, the principal factors influencing the corrosion process are the characteristics of the water of the system, the rate of water flow, the temperature of the system and the contact of dissimilar metals in the system. The variable characteristics of the water which determine its corrosiveness are its dissolved oxygen concentration, carbon dioxide content, pH and concentration of dissolved solids. Other factors may be involved, as, for example, the presence of free mineral acid, hydrogen sulfide, sulfur dioxide, and so forth.
The presence of oxygen dissolved in the water of a system is primarily the result of contact of the water with the atmosphere. The oxygen solubility in water is temperature and pressure dependent, with an increase in pressure increasing solubility, and with an increase in temperature lowering the oxygen solubility.
Corrosion produced by the presence of oxygen in the water of a system can take place in the form of small pits or depressions. As the corrosive process continues, these pits or depressions increase in area and depth and a nodule of corrosion products is formed. The corrosive attack is more severe when taking place in the form of pits or depressions since this permits deeper penetration of the metal and more rapid failure at these points.
2. Description of the Prior Art
Early efforts to reduce scale formation in water-carrying systems employed compounds such as tannins, modified lignins, algins, and other similar materials. However, use of these compounds entailed a number of disadvantages, including oxidation of the compounds in boiler systems, decomposition of the compounds on metal surfaces with resultant deposition of carbon, and the requirement of relatively large amounts of the compounds to achieve scale inhibition.
Chelating or sequestering agents have been employed to prevent precipitation or crystallization of scale-forming compounds. Such agents usually act by forming a water soluble complex with the cation constituent of the scale-forming compound, thus effectively inactivating the cation constituents so that their solubility product concentrations are not exceeded and precipitation or crystallization does not occur. However, it is inherently required that at least stoichiometric amounts of such chelants or complexing agents be employed, and generally it is necessary to use many times as much chelant as cation present. But, the use of such large amounts of treating agent is seldom desirable or economical. Moreover, proper application of qualified chelants involves optimum levels of suitable supplementary dispersing agents and the maintenance of favorable alkalinity as well as pH conditions.
Another type of agent which has been actively explored by the prior art as a scale inhibiting material is the threshold active inhibitor. Such materials are effective as scale inhibitors in amounts considerably less than that stoichiometrically required, and this amount is termed the threshold amount. The concept of threshold amounts will be further described hereinafter.
Inorganic polyphosphates have long been used as such threshold active inhibitors. For examples of such materials, see Fink and Richardson -- U.S. Pat. No. 2,358,222; Hatch -- U.S. Pat. No. 2,539,305; and Ralston -- U.S. Pat. No. 3,434,969.
A number of different polymeric materials have been previously employed as scale inhibiting agents. In particular, a number of polyacrylic, polymaleic, and polymaleic anhydride homopolymers and copolymers have found use. Johnson -- U.S. Pat. No. 2,723,956 discloses a method of reducing scale in boilers by incorporating in the boiler water-soluble copolymers of maleic anhydride and another polymerizable monoethylenic compound. Robertson -- U.S. Pat. No. 3,289,734 discloses a method of inhibiting scale formation on metal surfaces of a multiple-effect evaporator used for the processing of black liquor by treating the black liquor with a copolymer material comprised of alkane and maleic acid or anhydride units. Herbert et al. -- U.S. Pat. No. 3,293,152 discloses a method of preventing the formation of scale deposits on the heat transfer surfaces of apparatus used for the evaporation of sea water which consists of adding to the sea water a polyacrylic acid having a molecular weight of between about 20,000 and about 960,000. Engman et al. -- U.S. Pat. No. 3,516,910 discloses a method of inhibiting scale formation on metal surfaces of an evaporator used for the processing of black liquor by treating the black liquor with a water-soluble polymer which may be polymethacrylic acid, higher polyalkylacrylic acids, and copolymers and terpolymers of acrylic acid, methacrylic and higher alkylacrylic acids with each other and other vinyl monomers. These water soluble polymers may have a molecular weight from 1,000 up to 200,000. Jacklin -- U.S. Pat. No. 3,549,538 discloses a method of inhibiting and removing scale in boilers used to generate steam from water which comprises adding to the boiler water a composition consisting of a mixture of a nitrilo compound and a water soluble sulfoxy free polar addition polymer. In particular, the polar addition polymer may be a maleic anhydride polymer. Woodard -- U.S. Pat. No. 3,574,175 discloses copolymers of hydrochlorides of N,N-diallyglycinonitrile, N,N-diallyglycinamide, and N,N-diallyglycine with acrylic acid and acrylamide useful as scale formation inhibiting agents. Hwa et al. -- U.S. Pat. No. 3,578,589 discloses a method for inhibiting deposition of scale, mud, silt, sludge, and other foulants, in water cooled industrial heat exchange and water cooling systems which comprises adding to the cooling water a nonionic surface active agent and a water soluble polymer having a weight average molecular weight of at least 400 and selected from the group consisting of polyacrylic acid, polymethacrylic acid, and acrylic acid -- methacrylic acid copolymers. Rice et al. -- U.S. Pat. No. 3,589,998 discloses a method of inhibiting scale formation on the surface of membranes used in reverse osmosis water treatment which comprises adding to the input water a composition comprising the reaction product of acrylic acid, thioglycolic acid, and ammonium persulfate. King -- U.S. Pat. No. 3,617,577 discloses a method of inhibiting scale formation in aqueous systems by maintaining in such a system a threshold concentration of a linear ethylene-maleic acid copolymer having a molecular weight of from about 1,000 to 5,000. Johnson -- U.S. Pat. No. 3,715,307 discloses a method of treating water used in heat transfer equipment by adding to the feed water a water soluble, low molecular weight linear copolymer of maleic anhydride and a copolymerizable ethylenically unsaturated compound such as ethylene, vinyl acetate, acrylonitrile, acrylic acid, methyl vinyl ether, styrene and the like. In addition to the above disclosures, methods and materials for water clarification or purification wherein the compositions employed are similar to those discussed above, are set out in Johnson et al. -- U.S. Pat. No. 3,157,595; Fields et al. -- U.S. Pat. No. 3,398,092; Ryznar -- U.S. Pat. No. 3,492,226; and Fields et al. -- U.S. Pat. No. 3,554,935.
A variety of compositions have been employed in the art for the purpose of inhibiting corrosion of surfaces in water-carrying systems. Where the cause of the corrosion is dissolved oxygen, sodium sulfite is commonly used as an oxygen scavenging chemical deaerator. Catalytic aids for this process have also been developed. Hydrazine has been used as a reducing agent for the dissolved oxygen, giving only water and nitrogen as reaction products. Polyphosphates, for example sodium tripolyphosphate, are widely used in the treatment of once-through systems. Silicates, for example, sodium silicate, have also found acceptance.
Hatch et al. -- U.S. Pat. No. 3,483,133 discloses a corrosion inhibiting composition comprising aminomethylphosphonic acid compounds in combination with water soluble zinc salts. Oude Allnk -- U.S. Pat. No. 3,762,873 discloses a corrosion inhibiting method using substituted succinimides. Kerst -- Canadian Pat. No. 854,151 discloses a composition and method for inhibiting corrosion and/or the formation of calcium and magnesium containing scales where a combination of organophosphonic acid compounds and water soluble polymers having carboxyl or amide groups is employed. South African Pat. No. 71/7985 discloses a method of treating the water of an aqueous system with hydrolyzed polymaleic anhydride having a molecular weight of 300 to 5,000 for the purpose of inhibiting scale formation; while German Pat. No. 2,259,954 discloses the use of the same hydrolyzed polymaleic anhydride material in combination with a zinc salt for the purpose of inhibiting both corrosion and scale formation.